1. Field of Invention
This invention relates to the field of telecommunication, specifically to methods of building transmission systems.
2. Prior Art
One direction of improvement telecommunication systems is the development of methods of building transmission systems, which provide high level of utilization their transmission resources, especially for transmission of discontinuous, burst traffic created by plurality of simultaneously working sources of information. The most effective method used now for this purpose is statistical multiplexing. High efficiency of statistical multiplexing is achieved because of several features that distinguish it from other methods of multiplexing such as Time Division Multiplexing and Frequency Division Multiplexing. These features are:
Data presented as frames, packets, datagrams, or cells are transmitted through the transmission system at maximum transmission rate. This is why the statistical multiplexing has the shortest transmission time in contrast to other methods, where the frames are transferred using only part of transmission system's resources (time, frequency, and wavelength),
Every frame uses all transmission resources of transmission system but only for the time that is necessary for transmission. All other time the transmission resources are available for transmission of frames belonging to other messages. As a result, the transmission resources of the transmission system are distributed among frames coming simultaneously for transmission in the same direction in proportion to frames volumes. All other methods of multiplexing usually assign for every of simultaneously transferred frames only part of transmission resources. These provide in case of statistical multiplexing lowest than for other methods of multiplexing average time that the frame spends in the transmission system. This average time includes an average time of waiting transmission and an average time of transmission frames,
Depending on the requirements, every frame can be served with a different quality of service by applying to them different priorities.
Most transmission systems provide two-way communications by using two separate channels with equal bandwidth, each carrying data in one direction only. Applying statistical multiplexing for distribution transmission resources among streams of data sent in one direction to every channel independently allows achievement of high quality of service and high level of utilization of transmission resources in every direction of transmission. However, when the system is working, every moment it sends different volume of data in opposite directions and, as a result, provides different quality of service for data sent in opposite directions. In these systems, every channel in some periods can be overloaded, whereas the channel, which sends data in the opposite direction, in these periods can be empty, waiting for data.
Another problem that exists in every telecommunication network is associated with the fact that the resources necessary for transmission of data through every transmission line are determined at the period with the highest volume of traffic called the pick or rush hour. The time when the volume of traffic is high has usually length several hours a day only. At all other times less traffic is coming. As a result, the utilization of resources of transmission systems is low and unneeded consumption of energy takes place too.
At present, there are several know cases, where the same channel is used to carry signals in opposite directions simultaneously. One of them was developed and implemented in so-called local loops of public telephone networks, where for two-way voice and data transmission between a telephone set and a nearest central office one pair of copper cable is used. In this case, no sharing and possibility of distributing resources between opposite directions take place. Two electrical signals with similar features, sent simultaneously through one common channel but in opposite directions, do not interfere within the channel. The receivers receive them on the opposite sides of the channel without corruption (see Warren Hioki, Telecommunications, 3rd edition, 1998, pp 277-278). Another example, when one channel is used for sequential transmission of signals in opposite directions is half-duplex mode. In this mode every side of transmission system in turn get right to send waiting in buffer memory data and after this, transfer the right to send data to opposite side of transmission system. In the radio systems, the half-duplex mode allows using only one frequency for two-way communications. The half-duplex mode possesses two important futures. First, half-duplex mode uses one common channel for transmission in opposite directions. Second, distribution of transmission resources of the common channel depends on the volumes of traffic sent in opposite directions. This determines high flexibility of half-duplex mode. Half-duplex mode and statistical multiplexing have some common futures such as:
In both cases is used maximum transmission rate for transmission of every frame,
The time, for which frames of data use the transmission resources, depend on their volumes.
However, the statistical multiplexing and half-duplex mode also have one significant difference. In half-duplex mode, the resources are distributed between opposite directions. There are several downsides in the half-duplex mode such as the time of propagation of signals, switching time and times for reestablishing of bit synchronization for every change in direction of transmission. These components are the cause of loss efficiency for the half-duplex mode.
For better understanding peculiarities of half-duplex mode, on FIG. 1 is shown the structure of “point-to-point” transmission system that includes two similar transceivers 30 and 32 working in half-duplex mode and transmission media 34 that connects the transceivers and used for transmission signals between them. On FIG. 2 is depicted a time diagram of operation each transceiver. The time diagram shows a chain of periodically repeating sequence of operations. Every of them include transmission of data, propagation of data to opposite side, switching from reception to transmission on the opposite side, propagation of data from opposite side, reception of data, switching transceiver from reception to transmission. Combination of the propagation times, the switching times and, not shown on FIG. 2, times for reestablishing bit synchronization after every change of direction of transmission represent losses of efficiency half-duplex transmission systems. As can be seen from FIG. 2, the efficiency of the system depends on the combined times of transmission and reception for the same values of propagation and switching times. The bigger they are, the higher the efficiency of transmission system working in half-duplex mode. However, with the growth of transmission and reception times, the delay in data transmission will grow too. Because of this, fewer types of traffic can be sent through this transmission system since some types of traffic are not compatible with the increase and variations in delay. This is especially true about traffic that requires transmission in real time. Half-duplex mode cannot be effectively used for transmission of integrated traffic, because combined transmission and reception times have to be short for transmission of integrated traffic.
For some types of transmission systems, designers developed ways of improving efficiency in transmission systems while keeping their flexibility high. One example of the transmission system is GSM (Global System for Mobil communications) cellular system. There the time intervals of transmission and reception for every pair “base station—user station” are separated by the time intervals of communication between base station and others user stations. As a result, the loss in efficiency due to propagation and switching times are decreased.
Developers of the well-known LAN technology Ethernet considered the possibility that two transceivers without corruption can receive two signals sent through common transmission media by them and the fact of simultaneous transmission can be unrecognized. Because this is not acceptable for Ethernet technology, which used half-duplex mode, the developers established interdependency among values of 3 parameters of the system: the maximal length of cable, the minimal number of bits in frame, and the transmission rate. This allowed to ensure the 100% recognition of collisions in case of simultaneous transmission frames by two terminals (see Andrew S Tanenbaum, Computer Networks, 4th edition, 2002, pp 275-278). This technology allows easy and flexibly distribute transmission resources of the transmission system among connected terminals depending on their loads. However, in this case the efficiency transmission is low.
Within recent years, many solutions were developed that offer improvement of flexibility and efficiency mostly cellular or fiber optic transmission systems. However, no one of them offers a universal way of building effective and in the same time flexible transmission systems.